The present invention relates to an image forming apparatus such as an electrophotographic copier and, more particularly, to a method of controlling the surface potential of a photoconductive element for adequately controlling the contamination of the background area of the element ascribable to the increase in residual potential on the surface of the element and to the shaving of a photoconductive film provided on the element or similar cause.
A photoconductive element or image carrier for use in an electrophotographic copier or similar image forming apparatus usually has a photoconductive layer in the form of an organic semiconductor (OPC) on the surface thereof. Such a photoconductive element allows charge to accumulate thereon and thereby allows a potential to remain thereon due to fatigue as a copying cycle is repeated, despite that the surface of the element is discharged by light, for example, as well known in the art. The residual potential on the photoconductive element and, therefore, the potential in the background area of the element increases with the increase in the number of copies produced, i.e. the number of times that the copying cycle is repeated. When the residual potential increases to a given value, it causes the background area of the photoconductive element to be contaminated. The strength of the photoconductive film is relatively low and, depending on the conditions of use, the thickness is altered so that the sensitivity of the photoconductive element is changed. This is another cause of the contamination in the background area.
To eliminate the contamination ascribable to the increase in the residual potential as stated above, there has been proposed a method which senses the potential of the background area and, based on the sensed potential, adjusts one or more of the charge potential for charging the surface of the photoconductive element, the amount of light for illuminating the charged surface of the element, and the bias voltage applied to a developing unit which develops a latent image electrostatically formed on the element. For example, Japanese patent laid-open publication No. 201067/1984 discloses a method which senses the residual potential on the photoconductive element and corrects the bias potential and the amount of light on the basis of the sensed potential. Japanese patent laid-open publication No. 76546/1982 teaches a method which forms a toner image representative of a reference pattern having a reference density on the photoconductive element, generates a signal associated with the density of the toner image, and feeds it back to the charge potential and the amount of light. Japanese patent laid-open publication No. 191161/1988 shows and describes a method which compensates for the fatigue of the photoconductive element by controlling the charge potential and the amount of light in matching relation to the fatigue and idle time of the photoconductive element. Further, U.S. Pat. No. 4,870,460 discloses a method which discharges the residual potential on the surface of the photoconductive element except for the image area, develops the residual potential remaining after the discharge by a bias voltage which is lower than the bias voltage adapted for the reproduction of a document image, senses the density of the resulting visible pattern, and corrects, in response to the sensed density, at least one of the charge potential, exposing potential, and bias potential at the time of forming a document image. With any of these methods, it is possible to reproduce an image which has little suffered from the influence of background contamination.
However, the problem is that the contamination in the background area is derived from two different kinds of causes, i.e., the increase in the background potential due the residual charge, or residual potential, on the surface of the photoconductive element, and the change in the sensitivity of the element due to changes in the thickness of the OPC film or the like, as mentioned earlier. The two different kinds of causes each needs a different remedy. Specifically, when the residual charge accumulates, the background potential will not lower even if the amount of light is increased and, therefore, it is necessary to increase the charge potential and the bias potential for development to thereby lower the background potential. On the other hand, when the sensitivity of the photoconductive element is changed due to, for example, the changes in the thickness of the photoconductive film, the background potential will readily lower only if the amount of light is increased. Moreover, these two causes, in practice, increase the background potential in combination and thereby aggravate the complicated control over the background contamination. Another problem with the prior art implementations is that they simply adjust the amount of light, bias potential or charge potential in such a manner as to reproduce a predetermined image without making distinction between the different types of causes of the increase in background potential, failing to control the contamination satisfactorily.